Egg crate will block a percentage of light. So for a reef scenario it is not optimal. Screen is your best bet if you need a top. Look at bulkreefsupply.com they sell material to make your own screen top and pretty cheaply

Open top is the only way to get sufficient gas exchange. I buy standard window screens and use the clear plastic mesh you can get from BRS to make my screen tops. The clear mesh doesn't block hardly any light at all. A top will save fish from jumping and things accidentally falling into the water. Egg crate blocks too much light.